Wet oxidation is oxidation that takes place in the aqueous phase of organic and inorganic materials dissolved or suspended in an aqueous medium. This process occurs at an increased temperature and increased pressure in the presence of oxygen, which can be added in the form of a molecular oxygen, a gas containing ozone (e.g., air) or a solution that gives off oxygen (e.g., hydrogen peroxide). The purpose of wet oxidation is to reduce the chemical oxygen demand (COD) of the aqueous medium. To successfully conduct wet oxidation, two essential processes must be kept in mind.
The first process is a continuous transition of the oxygen from the gaseous into the liquid phase (mass transition). An adequate concentration of oxygen in the liquid phase must be maintained at all times, so that the wet oxidation reaction can proceed. The importance of the continuous transition of oxygen from the gaseous into the liquid phase is best illustrated by comparing the equilibrium concentration of oxygen in pure water at the temperatures and pressures usual during wet oxidation to the typical COD values of an aqueous medium to be treated by wet oxidation: The equilibrium concentration equals less than 1 g/l, while the typical COD value is in the range of 30 to 100 g/l. This means that in a wet oxidation system, a quantity of oxygen at least 30 times as large as the equilibrium must normally pass from the gaseous into the liquid phase to obtain the desired COD reduction. An efficient wet oxidation system must therefore constantly maintain a large zone for gas/liquid transition as well as high gas/liquid transition coefficients in the total reactor volume. Means for efficient dispersion, for breaking and distributing the gas bubbles in the total reactor volume, are thus of central importance in a wet oxidation system. If such means do not exist, zones undersupplied with oxygen will form inside the wet oxidation reactor, especially if the oxidation reaction proceeds rapidly and the dissolved oxygen is quickly consumed. Because wet oxidation will not proceed in zones undersupplied with oxygen, the result is a reduction in the effective reactor volume.
The second important process that must be taken into account is the inherent kinetic process of the oxidation reaction itself. The oxidation rate, as reported in the literature (cf. Li et al., AlChE Journal, Vol. 37, No. 11, 1991, pp. 1687-1697), depends in the first order both on the concentration of dissolved organic and oxidizable inorganic materials and on the concentration of dissolved oxygen. The expert can easily grasp that the positive order of oxidation kinetics requires the use of reactors with a low degree of back-mixing. In wet oxidation, the degree of the back-mixing in the liquid phase is especially important, because it is in the liquid phase that the organic and oxidizable inorganic material is present and the oxidation reaction takes place. A low degree of back-mixing means that a smaller total reactor volume is required to attain the desired degree of oxidation of these organic and oxidizable inorganic materials.
In summary, it can be said that an efficient gas/liquid mass transition (gaseous oxygen into the aqueous phase) in a reactor that is operated with a low degree of back-mixing in the liquid phase permits a smaller and more compact oxidation reactor to achieve a desired COD value. In light of the high pressure used during wet oxidation, this represents a remarkable advantage in comparison to less efficient systems.
A relatively effective system for gas/liquid transition is proposed in U.S. Pat. No. 3,870,631. That document describes the use of strong mixing to attain efficient contact between the aqueous phase and the gaseous phase containing oxygen. Mixing is attained by rapidly rotating stirring mechanisms, which are driven from outside of the reactor. The energy consumption and difficulties in sealing the drive shafts are significant disadvantages of this system.
U.S. Pat. No. 4,793,919 describes an alternative that consumes less energy. This alternative calls for an apparatus with a pump for circulating the aqueous suspension of the material to be oxidized through an arrangement of static mixers. The static mixers comprise a large number of guiding devices arranged in the reaction zone. Along with this reaction zone, the apparatus has a zone for the internal recirculation of the mixture. In this recirculation zone, there is a much less intensive mass transition regime (gas/liquid), because the recirculation zone contains no static mixing devices. The internal circulation of the mixture is maintained by the pump and by the buoyancy forces created by introducing a gas that contains oxygen into a zone of the reactor separate from that into which the aqueous suspension is introduced.
In the two systems described above, the intensive mixing and internal recirculation needed to improve the mass transition characteristics of the system inevitably result in a high degree of back-mixing in the aqueous phase. This in turn leads to large reactor volume, which is required for the desired reduction of the COD value. In the past, the solution to this problem has usually been seen in connecting two or more reactors in series, one behind the other, with a high degree of back-mixing. Such an arrangement is also proposed in the two aforementioned patents.